agg_null_rec_1.3.query.sqlpp

[ASTERIXDB-2459][FUN] Add skewness and kurtosis aggregate function
- user model changes: yes
- storage format changes: no
- interface changes: no

Details:
1. Add skewness and kurtosis function and test cases
2. Add skewness and kurtosis distinct functions
3. Add documentation for skewness and kurtosis
4. Cleaned up some files related to the stddev and var tests cases
5. Change stddev() to stddev_samp() and var() to var_samp()

Change-Id: Iae0d7fc9d5ab3676b23b27ad62a730598221d8a5
Reviewed-on: https://asterix-gerrit.ics.uci.edu/3037
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Contrib: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Dmitry Lychagin <dmitry.lychagin@couchbase.com>